1. Field
The present embodiments relate to a thin film transistor, and more particularly, to a thin film transistor including an HfInZnO-based oxide semiconductor layer as a channel layer, a method of manufacturing the same, and an organic electroluminescent device including the thin film transistor.
2. Description of the Related Technology
A thin film transistor (TFT) is a type of a field effect transistor made by depositing thin films of a semiconductor material over an insulating supporting substrate. The TFT basically includes three terminals, e.g., a gate, a drain, and a source, and mainly performs a switching operation. In the switching operation of the TFT, a voltage applied to the gate is adjusted to set a current flowing between the source and the drain in an on-state or off-state. TFTs are used in sensors, memory devices, optical devices, and the like, and are most widely used in pixel switching devices of flat panel displays.
Currently, commercially available products, such as notebook computers, PC monitors, TVs, mobile devices, and the like mostly include amorphous silicon thin film transistors (a-Si TFTs). The atom arrangement in amorphous silicon is not as regular as in crystalline silicon, and only short range order is present. As amorphous silicon may be easily deposited over large areas (such as a glass substrate) at low temperatures, amorphous silicon is most widely used in TFTs. However, as the requirements for display devices with larger sizes and higher image quality have increased, high performance thin film transistors having higher electron mobility than a-Si TFTs, e.g., 0.5 to 1 cm2/Vs, and the manufacturing techniques have been required.
Poly-Si TFTs have superior performance to a-Si TFTs. Polysilicon (poly-Si) TFTs have a mobility of several tens to hundreds of cm2/Vs, and thus a data driving circuit or periphery circuit required for high mobility may be embedded in a substrate. In addition, channels of such TFTs may be made short, and thus an aperture ratio of a screen may be high. Also, since the driving circuit is embedded in the substrate, there are no pitch limitations in wiring for connecting the driving circuit according to an increased number of pixels, and thus high resolution may be realized, a turn-on voltage and power consumption may be reduced, and the poly-Si TFTs may have less characteristic deterioration.
However, the crystallization process used to manufacture poly-Si TFTs is complicated, and thus the manufacturing costs may increase. In addition, due to technical problems such as manufacturing equipment limitations or uniformity defects, manufacturing of a large-scale substrate using poly-Si TFTs has not been realized up to date.
Therefore, research has been actively conducted on novel TFTs having advantages of both a-Si TFTs and poly-Si TFTs. Oxide semiconductor devices are representative examples of the TFTs.
Oxide semiconductor devices may be categorized into crystalline oxide semiconductor devices including a crystalline oxide such as ZnO and amorphous oxide semiconductor devices including an amorphous oxide such as GIZO (GaInZnO). The amorphous oxide semiconductor devices may be manufactured at a low temperature, easily be made in large sizes, and have high mobility and an excellent electric characteristic like the poly-Si TFT. Thus, research is currently being conducted in order to use an oxide semiconductor layer in a channel area of a TFT. In an oxide semiconductor, since the overlapping of a wave function of an s-orbital where electrons that are the least anisotropic according to a direction among the outermost electrons of a metal reside contributes to the band charge transfer of electrons, it is considered that an amorphous thin film fanned of the oxide semiconductor may have a high mobility, such as, 10 cm2/V·s or greater.
However, recent reports have disclosed that the characteristics of commonly used InGaZnO oxide semiconductor devices deteriorate when exposed to plasma or external agents (for example, moisture, oxygen, or the like). Also, to protect the oxide semiconductor layer, an etch stop layer is formed on the oxide semiconductor layer. However, depending on the conditions for forming the etch stop layer, the characteristics of the oxide semiconductor devices may severely deteriorate. In addition, the condition ranges wherein the characteristics of the oxide semiconductor devices do not deteriorate are limited, and thus there is a need for fundamental changes in the oxide semiconductor devices.